The hypothesis of this study is that moderate hyperhomocysteinemia causes endothelial dysfunction in humans. We will examine subjects with and without conventional risk factors for atherosclerosis in order to assess whether moderate hyperhomocysteinemia has deleterious endothelial effects alone or only in combination with other risk factors known to impair endothelial function. Endothelial function will be tested in conduit vessels using flow mediated vasodilatation and in resistance vessels using brachial artery administration of drugs to cause endothelium- dependent vasodilatation and to inhibit generation of nitric oxide.